This invention relates to the field of charging devices as for nickel-cadium batteries and, more particularly, to the prevention of excessive over-charging under any set of battery conditions.
It is well known that over-charging of an already charged battery or charging of a cold battery can cause gassing, electrolyte venting, loss of battery capacity and actual physical damage. Prior art protection circuits have typically provided an absolute voltage clamp which is set, for example, at the gassing voltage of approximately 1.6 volts per cell. However, batteries vary greatly due to a number of parameters such as age, temperature and memorization, plus the variations between manufacturers and even different manufacturing lots from any one source. An absolute voltage clamp can prevent gassing due to a cold battery (T&lt;+5.degree. C.) but will not provide protection against over-charging, particularly, if a cold and fully-charged battery should be inadvertently put on charge. This is a relatively frequent occurrence in a two-way communications system where the portable units are routinely put into a charging unit at the end of the user's tour of duty; e.g., in police, fire, railroad yard service, etc. Another protection circuit, used for a high-charging rate mode, charges a timing capacitor and when a predetermined voltage appears on the capacitor, the charging of the battery is stopped. In this mode, all other controls such as temperature and current are overridden unless the predetermined voltage is dangerously high.